Add a mechanism to allow interval timer cancellation

[barbu110]

Interval timers could be used for creating a retry mechanism. As an example, see the following scenario:

• our callback function does some operation that might fail
• we start an interval timer with microloop::timers::set_interval with callback callback to repeat the operation every X seconds until it succeeds.

Currently the until it succeeds part cannot be satisfied by the current API because the callback has no way of stopping the timer.

Changes in the API:

• A TimerController class should be implemented with a cancel method.
• The Timer callback should be called with this sole argument, an instance of TimerController constructed when the Timer is constructed.